1. Field of the Invention
The present invention relates to redundancy for a process data collection system to acquire process data (including alarm data and event data) obtained in a process control system and to provide those data to a client as production control information.
2. Description of the Prior Art
There are conventional systems in which a process data collection system to acquire process data (including alarm data and event data) obtained in a process control system and to provide those data to a client as production control data is made redundant.
FIG. 1 is a functional block diagram indicating an example of the configuration of conventional redundant process data collection systems. First, the process control system is a distributed control system having a hierarchical structure, in which upper level human interface station (hereinafter called “HIS”) 1 and field control station (hereinafter called “FCS”) 3 distributed in plants are connected to control bus 2.
FCS3 is in charge of control for a plurality of field instruments 51, 52, . . . 5n via I/O bus 4 and gives process data (including alarm data and event data) 6 obtained as the result of control to HIS1 via communications. In HIS1, operations and monitoring are performed based on the given data.
Numeral 7 shows a real server connected to control bus 2, which has a function to provide process data 6 from FCS3 to the upper level user (hereinafter called “client”) side that utilizes process data as production control information.
Real server 7 is made redundant by real server#1 71 and real server#2 72 to secure reliability in providing information to the client side. Process data 6 are supplied to both real server#1 71 and real server#2 72 in common and thus information simultaneity and equivalency are ensured.
These real server#1 71 and real server#2 72 have interfaces in accordance with OPC (OLE for Process Control) Foundation standards which is made open as the common interface for process data reference (hereinafter called “OPC interface”).
Numeral 8 shows a virtual server having an OPC interface, which is connected to real server#1 71 and real server#2 72 via general communication bus 9 represented by Ethernet™. This virtual server 8 intermediates between an upper level client and real server#1 71/real server#2 72 and serves to make the real servers appear to be a single real server when viewed from the client side.
Virtual server 8 has switch-over means 81 and monitoring means 82. Switch-over means 81 is provided with a two-pole toggle switch function, and selects signals from either the primary contact P to which information d1 from real server#1 71 is input via virtual client#1 83 or the secondary contact S to which information d2 from real server#2 72 is input via virtual client#2 84, and notifies OPC client 10 of selecting information d0.
In the normal condition, switch-over means 81 selects the primary contact P, and so real server#1 71 is set as the control server and real server#2 72 is set as the standby server, and process data acquired by real control server#1 71 are given to OPC client 10.
Monitoring means 82 monitors the following abnormalities using diagnosis based on periodical calling and statuses of communication condition and executes switch-over operation which replaces the control server with the standby server by sending a switch-over command m when an abnormality occurs:    (1) Loss of real OPC server function due to a hardware failure or software failure in the real OPC server    (2) Communication error due to a network failure between the virtual server and the real serverSymbols “w1” and “w2” indicate diagnostic response data based on periodical calling and symbol “n” indicates the network status confirmation signal.
OPC client 10 consists of a real-time database that holds process data from the real server, received via virtual server 8, in real-time for a prescribed time interval; a historical database where process data from the real-time database are acquired periodically and are processed to long-term trend information; and client applications that call information in those databases and utilize them.                [Patent Document 1]        Gazette for Japanese Laid-open Patent Application No. 2000-278297        
Conventional redundant process data collection systems having the above-described configuration have the following problems:    (1) Since even a real server set as the standby server out of two real servers accesses the process data, communication loads for collecting data which are not used normally are generated.    (2) Since all the data items requested by a client are periodically updated evenly, unnecessary duplication due to individual executions of the same data collection takes place and thus the communication load on the control bus increases.    (3) The occurrence of missing process data, caused by a loss of OPC client function due to a hardware failure or software failure and maintenance in the OPC client, cannot be backed up.